Illuminated display panels are known and commonly used for providing visual information relating to electronic equipment in general, and in particular for providing motor vehicle occupants with information relating to engine/vehicle operating conditions, climate conditions and control thereof, entertainment control, and other automotive-related functions. Early display illumination techniques involved backlighting or side-lighting a mechanically formed panel, typically via one or more incandescent lamps, wherein the panel defined a predetermined message. For example, early automotive radios typically included a mechanically actuated frequency tuning panel that defined AM and FM frequency bands. For night driving, one or more incandescent lamps were provided that served to sidelight the frequency tuning panel for increased visibility thereof. As such radios became more sophisticated and consequently included more functions, radio panels were typically formed with transparent or translucent messages that were back-lit at night to provide for visibility of such messages.
As automotive radios continued in their evolution, electronic tuning systems were introduced and alpha-numeric displays became commonplace, wherein light emitting diode (LED) systems were typically used to provide saturated (i.e., radiation which is substantially pure in color and only has a small range of spectral frequencies) alpha-numeric information relating to the presently tuned frequency. LED illumination systems in radio and other electronic systems, particularly in automotive applications, were typically replaced by vacuum fluorescent (VF) displays due to the excellent contrast and light intensities provided by such display systems.
Concomitant with advancing information technology is an increasing need to provide displays with larger arrays of pixels to produce more sophisticated images. However, large pixel arrays require high voltage power supplies, highly multiplexed VF drive schemes with expensive driver circuitry, and which tend to generate significant amounts of radio frequency interference (RFI). In an attempt to decrease display system cost and other problems associated with large pixel array VF display systems, some display system designers have begun to substitute such display systems with liquid crystal display (LCD) systems.
Most known and commercially available LCDs operate by rotating the polarization angle of light that passes through the display elements in response to a voltage impressed across each of the various display elements. Polarizing films are typically laminated to each side of a display cell, and when light passes through the first polarizer it is rotated to some degree by the liquid crystal fluid. The second polarizer then attenuates the light by an amount proportional to the alignment of the polarization angle of the light relative to the axis of the polarizing film. The resulting contrast of the LCD is the ratio of the light flux through the most transmissive state of the display elements to the light flux through the least transmissive state. Since the least transmissive state is the denominator of the contrast ratio, it dominates the ability to achieve high contrast. The least transmissive state, or "dark" state also defines a dark state color hue of the display that is easily perceived by an observer when there are many dark elements or segments, or when the background state of the display is dark. The dark state of any LCD element or background is defined as a state wherein the light impinging upon the element or background is rotated by the liquid crystal such that the polarization angle of the light reaching the second polarizer is exactly 90 degrees relative to its polarization axis, whereas the opposite light state of any LCD element or background is defined as a state wherein the light impinging upon the second polarizer is parallel with its polarization axis. In-between states; i.e., operational states between the light and dark states, are also possible as is known in the art.
In order to provide for a seamless replacement of VF displays with LCDs, the LCDs must be backlit in such a manner so as to simulate, at least as closely as possible, the blue-green color associated with typical VF display systems. Heretofore, blue colors were difficult to produce with incandescent lamps since the frequency spectrum emitted by such lamps typically contains negligible content in the blue frequency range. However, with the advent of higher frequency LED colors, it is possible to produce blue-greet VF backlight colors.
Conventional LEDs produce light that is substantially "saturated"; i.e. light that is pure in color and includes only a small range of spectral frequencies. In LCD back-lighting applications that require saturated light, one or more conventional LEDs may be provided to produce the desired illumination. In cases where the desired saturated color cannot be produced by a single LED, two or more LEDs of different colors may be chosen, whereby the ratio of intensities produced by the various LEDs may be controlled such that the resultant color is perceived to be nearly that of the target color. However, the blue-green color of a typical VF display is a "desaturated" color; i.e., color that is unpure and includes a broader range of spectral frequencies than that of a saturated color. As will be described in greater detail hereinafter, the mixing of saturated LED colors to produce a desaturated target color, such as the blue-green color associated with typical VF displays, can lead to disturbing visual effects when the resultant light interacts with a LCD.
Liquid crystal displays are known to be dispersive systems in that the degree that light is rotated through display elements for a given impressed voltage varies somewhat as a function of color. With saturated light, such dispersion is minimal in the light state as well as in between light and dark states, and when saturated light pass through a display element in the dark state, nearly all of the light will be attenuated depending upon its purity. However, when desaturated light (i.e., light containing a wide range of colors) passes through a display element, such dispersion occurs in the light and dark states, as well as in-between states. For example, in the light state, different desaturated colors will pass through the LCD elements or background at slightly different angles relative to the polarization axis of the second polarizer. Conversely, in the dark state, some colors will be highly attenuated while others will not be rotated to be exactly perpendicular to the axis of the second polarizer. Light colors that are not exactly perpendicular to the axis of the second polarizer will, to some extent, pass through the display element in its dark state. The contrast for those colors will be noticeably reduced; i.e., the dark segments will have less contrast for desaturated colors than for saturated colors, and will further have a hue corresponding to the colors that leak through the polarizer.
The degree that light is rotated through a display element also depends on the viewing angle relative to the display. In general, variations in the rotation of the polarization angle with respect to off-normal viewing angles relative to the display are not symmetric and can be quite pronounced, wherein such variations are typically very dispersive in nature.
Heretofore, desaturated colors for LCD back-lighting have been produced by mixing light emitted by saturated LEDs (i.e., spectrally "pure" colors) in proportions such that the resultant color matches the target backlight color. One known system 10 for back-lighting a LCD unit via a mixture of saturated LEDs is illustrated in FIG. 1. Referring to FIG. 1, a side elevational view of a known LCD unit 12 is shown including a back-lighting unit 14 having a number, N, of saturated LEDs 16.sub.1 -16.sub.N suitably mounted to emit radiation toward a back panel of LCD unit 12. In system 10, the LEDs 16.sub.1 -16.sub.N may, for example, be chosen to be two groups of LEDs each emitting different saturated colors. The two colors are typically chosen so that they lie on a line on a known CIE chromaticity diagram (hereinafter "CIE color chart") that contains therebetween the coordinates of the target color. The ratio of the distance from the target color to the two colors of the associated with the two groups of LEDs is monotonically related to the intensity of light needed by each group in order to properly mix to form the target color. An example of such a CIE color chart illustrating a known technique for producing a blue-green VF-type light color using two sets of different color saturated light producing LEDs is illustrated in FIG. 2.
Referring to FIG. 2, a color boundary 30 identifies pure saturated frequencies, and the desaturated colors are found more toward the center of the bounded region. Most LED colors lie close to boundary 30, and to produce a desaturated color near the middle of the bounded region using saturated LEDs, at least two saturated LED colors must be chosen that are on substantially opposite sides of the chart, wherein the two colors are typically in chromatic contrast with each other. In the example illustrated in FIG. 2, a number of red LEDs and a number of blue-green LEDs are chosen to produce a resultant blue-green color closely resembling a VF display, wherein the red and blue-green LEDs each produce saturated radiation. The color coordinates of each set of LEDs operated independently establish the endpoints of the color line (marked RED LED and BLUEGREEN LED), wherein the target color corresponds to x=0.26, y=0.45. The coordinates of the mixture (marked RESULTANT COLOR) is extremely close to the target color, wherein the mixture illustrated in FIG. 2 was achieved by adjusting the current through each of the LEDs; i.e., by adjusting the ratio of the number of red LEDs to the number of blue-green LEDs. This known adjustment techniques allows the resultant color coordinate to be manipulated along the color line that connects the blue-green and red LEDs, thereby enabling close matching of the color mixture with the target color. In FIG. 2, the RESULTANT color was achieved by running a current of 5 mA through 2 red LEDs and 18 mA through 4 blue-green LEDs.
Since the frequencies of LED colors from opposite sides of the boundary 30 vary significantly from each other, dispersion through the LCD can consequently be quite large. The net effect of using such a light mixture for back-lighting a LCD is that the color of the various segments, particularly in their light state, may noticeably vary at different viewing angles relative to an angle normal to the display. For example, the light from LEDs of one color may pass through the element or background in one direction while the light from LEDs of the other color may pass through in a different direction. In some situations, the variations in viewing angle from one eye to the other may be such that each eye perceives a different hue, wherein such a condition can be very disturbing to the viewer. The goal of maintaining color harmony in motor vehicle applications is frustrated when displays appear to be different colors when viewed from different angles.
A primary reason for this phenomenon may be seen with reference to FIG. 3 which illustrates a spectral waveform 32 of relative light intensity vs. wavelength for the red/blue-green LED mixture of FIG. 2. Referring to FIG. 3, waveform 32 indicates two distinct spectral peaks 34 (corresponding to the blue-green LEDs emitting light at approximately 500 nm) and 36 (corresponding to the red LEDs emitting light at approximately 640 nm). Although light emitted by the red and blue-green LEDs is suitably mixed prior to transmission through the LCD polarizers and the resultant color therefore closely resembles the characteristic blue-green color of typical VF displays when the LCD is viewed perpendicular to its display, each of the dominant peaks 34 and 36 are very discernable when the display is viewed at off-angles. For example, referring to FIG. 4 which shows perspective view of a group of display segments 18 of a LCD unit 12, the red/blue-green LED mixture of FIG. 2 may produce the characteristic blue-green color of a typical VF display when viewed along axis 20 normal to the display. However, if LCD unit 12 comprises a portion of an automotive radio display positioned in the center of the dash, the segment group 18 may appear bluish to the driver viewing the display at an angle ( relative to axis 20 (e.g., along axis 22), but may appear reddish to a passenger viewing the display at an angle .beta. relative to axis 20 (e.g., along axis 24).
One way to reduce the foregoing color variation problem is to mix three or more LED colors to form a resultant desaturated color. However, as the number of colors increases, so to does the ability to accurately control the intensities of the component colors over normal operating temperatures to thereby maintain a target color. Moreover, tuning the mixture of three or more colors during component manufacture to compensate for differences in the light production efficiencies of individual LEDs may be prohibitively difficult.
Cost is typically a significant design factor when utilizing LEDs to back-light LCD units. In terms of lumens per dollar, LEDs emitting longer wavelengths (e.g., red to yellow) are currently the least expensive while LEDs emitting shorter wavelengths (e.g., blue-green) are significantly more costly by comparison. Accordingly, it is commonplace to design LED-based LCD backlighting systems for simulating VF display colors by using as many of the longer wavelength LEDs as possible and then pulling the color coordinates to the target color value by using the least amount of the shorter wavelength LEDs. This conventional approach typically leads to the use of red and yellow LEDs to produce most of the lumens for desaturated target colors while adding as few as possible blue-green LEDs to achieve the desired target coordinates. This approach, however, also leads to the various problems and undesirable effects described hereinabove. What is therefore needed is an improved LED-based LCD back-lighting system that does not suffer from the drawbacks of known LED-based LCD back-lighting systems. Such an improved system should preferably be both cost efficient and simple to manufacture.